Process for making silicon nitride cutting tool material

ABSTRACT

The ceramic cutting tool material according to the present invention comprises a beta silicon nitride matrix with total amount of 0.5-10 weight %, preferably 0.5-6 weight %, of an intergranular phase and 0.05-3 weight % of at least one secondary crystalline phase of a transition metal carbide, nitride, carbonitride and/or silicide present as spherical particles with a size of 0.1-2 μm, preferably submicron (0.01-1 μm). The transition metal is preferably niobium and/or tantalum. The material has less than 1 volume %, preferably less than 0.3 volume %, porosity. The beta silicon nitride grains are to at least 10%, preferably more than 20%, elongated with an aspect ratio greater than 3, preferably greater than 5. The grain diameter of the beta silicon nitride grains is in the range of 0.2-10 μm, preferably 0.2-5 μm, and most preferably 0.2-3 μm.

This application is a divisional of Application Ser. No. 08/880,297,filed June 24, 1997, now U.S. Pat. No. 5,914,286.

BACKGROUND OF THE INVENTION

The present invention relates to a silicon nitride cutting tool withimproved properties for metal cutting applications in cast iron.

Silicon nitride has been recognized as a cutting tool material formachining of cast iron due to its good wear resistance and good hightemperature properties. During the last decade, the development of thematerial has led to an increased use in metal cutting applications andwith more advanced silicon nitride material with improved properties,the potential will grow.

In U.S. Pat. No. 5,382,273, a silicon nitride based ceramic material fora cutting tool is described. The material is a beta silicon nitride withless than 5 weight % of an intergranular phase consisting of magnesiaand yttria. This material is disclosed to have improved metal cuttingperformance, improved hardness at 1000° C., good transverse rupturestrength and improved Weibull modulus compared to prior art.

Pyzik et al. have described in a number of patents a self-reinforcedsilicon nitride that exhibits high fracture toughness and high fracturestrength. In U.S. Pat. No. 5,312,785, a process of producing aself-reinforced silicon nitride comprising a glassy phase, a secondcrystalline phase of zirconium oxide and a crystalline phase of a metalzirconium silicide and/or metal zirconium silicon nitride is described.In U.S. Pat. No. 5,160,508, a self-reinforced silicon nitride ceramic ofhigh fracture toughness is described. The material contains a i)crystalline beta silicon nitride phase with at least 20 volume % of thebeta silicon nitride in the form of whiskers having an aspect ratiogreater than 2.5, a glassy second phase containing ii) densificationaid; iii) conversion aid; iv) an aid which enhances the growth of betasilicon nitride whisker; and v) silica. U.S. Pat. No. 5,120,328describes a method of manufacturing dense self-reinforced siliconnitride by pressureless or low pressure gas sintering. The siliconnitride body comprises at least 20 volume % beta silicon nitridewhiskers, 2-10 weight % of a glassy phase, 0.5-5 weight % of a secondcrystalline phase of zirconium oxide and optionally 0.1-3 weight % of acrystalline phase of a metal zirconium silicide and/or metal zirconiumsilicon nitride. In U.S. Pat. No. 5,118,645, a process of preparing asilicon nitride body from a powder mixture of silicon nitride anddensification, conversion, whisker enhancing and Palmqvist toughnessenhancing aids is described. U.S. Pat. No. 5,091,347 describes a processfor preparing a silicon nitride from a mixture of i) silicon nitride andii) silicon dioxide as densification aid; iii) a conversion aid; and iv)a whisker growth enhancing aid at sintering temperatures above 1750° C.and pressures of at least 20.7 MPa.

In U.S. Pat. No. 4,497,228, an abrasion resistant silicon nitride isdescribed. This is achieved by adding up to 60 volume % of hardparticles of refractory metal carbides and nitrides or combinationsthereof.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to avoid or alleviate the problems ofthe prior art.

It is the object of this invention to show that by using a relativelysmall amount of sintering aids and by in situ formation of secondarycrystalline phases of carbide, nitride, carbonitrides or suicides thatlower the amount of glassy intergranular phase in the final product andincrease the wear resistance, a material with good fracture toughness,thermoshock resistance and wear resistance can be manufactured.

In one aspect of the invention there is provided a silicon nitridecutting tool insert comprising a beta silicon nitride matrix with atotal amount of 0.5-10 weight % of an intergranular phase and 0.05-3weight % of at least one in situ formed secondary crystalline phase of atransition metal carbide, nitride, carbonitride or silicide present asspherical particles with a size of 0.1-2 μm and wherein the beta siliconnitride grains are to at least 10% elongated with an aspect ratiogreater than 3, with a grain diameter in the range of 0.2-10 μm and lessthan 1 volume % porosity.

In another aspect of the invention there is provided a silicon nitridecutting tool insert comprising a beta silicon nitride matrix with atotal amount of 0.5-6 weight % of an intergranular phase and 0.05-3weight % of at least one in situ formed secondary crystalline phase of atransition metal carbide, nitride, carbonitride or silicide present asspherical particles with a size of submicron (0.01-1 μm) and wherein thebeta silicon nitride grains are to at least 20% elongated with an aspectratio greater than 5, with a grain diameter in the range of 0.2-5 μm andless than 0.3 volume % porosity.

In yet another aspect of the invention there is provided a method ofmaking a silicon nitride cutting tool insert by powder metallurgicalmethods comprising preparing a silicon nitride slurry by wet dispersionin water or an organic solvent of silicon nitride powder with powders ofyttrium oxide 0.1-5 weight %, aluminum oxide 0.1-5 weight % andtransition metal oxides, prefeably niobium oxide or tantalum oxide ormixtures thereof in an amount of 0.1-5 weight % whereby the total sum ofadded oxides is less than 6 weight % and dispersing agents optionallytogether with suitable pressing aids whereafter the slurry is dried andgranulated to a powder which is formed to a cutting tool insert of adesired shape and sintered using a pressure assisted sinteringtechnique.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows a transmission electron microscopy (TEM) micrograph inabout 10,000× magnification of the microstructure of a silicon nitridematerial according to the invention. The grey spherical and elongatedgrains are beta Si₃ N₄ and the black spherical grains are NbSi₂.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention relates to a silicon nitride based cutting toolinsert for chipforming machining of cast iron material having improvedmechanical, chemical and wear properties. This is obtained by additionof one or more transition metal oxides from Group IVa, Va and VIa, whichact as a sintering aid together with alumina and yttria and also promotethe growth of elongated grains of beta silicon nitride. The transitionmetal also forms small particles of a secondary crystalline phase oftransition metal-nitride, -carbide, -carbonitride or -silicide duringsintering.

The ceramic cutting tool material according to the present inventioncomprises a beta silicon nitride matrix with a total amount of 0.5-10weight %, preferably 0.5-6 weight %, most preferably 0.5-4 weight %, ofan intergranular phase and 0.05-3 weight %, preferably 0.3-2 weight %,of at least one secondary in situ formed crystalline phase of atransition metal carbide, nitride, carbonitride and/or silicide,preferably a carbonitride and/or silicide, present as sphericalparticles with a size of 0.1-2 μm, preferably submicron (0.01-1 μm). Thetransition metal is preferably niobium and/or tantalum. The material hasless than 1 volume %, preferably less than 0.3 volume %, porosity. Theintergranular phase can be regarded as an amorphous phase with avariable composition depending on the raw materials. It is oxide basedbut it can also to some extent dissolve nitrogen and carbon. The betasilicon nitride grains are to at least 10%, preferably more than 20%,elongated with an aspect ratio greater than 3, preferably greater than5. The grain diameter of the beta silicon nitride grains are in therange of 0.2-10 μm, preferably 0.2-5 μm, and most preferably 0.2-3 μm.

The material according to the invention is manufactured by powderprocessing followed by sintering. A silicon nitride slurry ismanufactured by wet dispersion of the silicon nitride together withsuitable amounts of additives in water or an organic solvent. Theadditives for the intergranular phase are yttrium oxide 0.1-5 weight %,preferably 0.2-3 weight %, most preferably 0.5-2 weight %, aluminumoxide 0.1-5 weight %, preferably 0.2-3 weight %, most preferably 0.2-2weight %, and transition metal oxides, preferably niobium oxide ortantalum oxide or mixtures thereof in amount of 0.1-5 weight %,preferably 0.2-3 weight %, most preferably 0.5-2 weight %. In somecases, SiO₂ is also added in an amount less than 1 weight %, preferably0.1-0.7 weight %. The total sum of added oxides should however,preferably be less than 6 weight %, and most preferably less than 4weight %, however it should be more than 0.5 weight %, preferably morethan 1 weight %. The added aluminum oxide should preferably be presentin the intergranular phase and not form a solid solution with thesilicon nitride. It might also be possible to use other transition metalcompounds that in situ will form small spherical particles of nitrides,carbides, carbonitrides or silicides that also increase the wearresistance. Suitable dispersing agents are added possibly together withsuitable pressing aids (organic additives). The slurry is then dried andgranulated.

The granulated powder is then formed into a body with a shape and a sizewhich, after sintering, will result in the desired shape and size usingeither uniaxial pressing or isostatic pressing. The relatively lowamount of intergranular phase makes it necessary to use a pressureassisted sintering technique such as hot press (HP), gas pressuresintering (GPS) or hot isostatic pressure (HIP). The amount of formedintergranular glassy phase should however, be high enough so that thematerial reaches essentially full density during the chosen sinteringprocess. This applies in particular, to the GPS sintering process. Whenhot press technique (uniaxial pressure sintering) is used, no pressingaids are needed and the granulated powder is filled into a hot pressgraphite die and then hot pressed. In the case of GPS and HIP technique,pressing aids are usually used and they are removed by a heat treatmentat a temperature in the range of 400-800° C. and in a suitable heattreatment atmosphere, preferably hydrogen or vacuum. The sinteringtemperature and sintering pressure depend upon which sintering techniqueis used. When hot pressing, a sintering temperature between 1600°-2000°C. is required and the sintering pressure is in the range of 20-50 MPa.When HIP sintering, a sintering temperature between 1500°-1800° C. usrequired and the gas pressure is in the range of 50-200 MPa. When GPSsintering, a sintering temperature between 1600°-2000° C. is normallyrequired and the gas pressure is in the range of 5-20 MPa, preferably8-12 MPa.

During the sintering of the material, a crystalline phase or phases oftransition metal carbide, nitride, carbonitride or silicide is nucleatedfrom the intergranular phase and thus formed in situ. The carbide can beformed due to the existence of carbon monoxide in the furnace atmosphereoriginating from the graphite parts and residual oxygen or oxides in thefurnace. It is also possible to use gas mixtures of nitrogen and carbonmonoxide. The crystalline phases reduce the amount of the glassyintergranular phase and formation of a relatively small amount of atleast one in situ formed crystalline transition metal compound of thistype has now surprisingly been found to increase the wear resistance ofthe final material. In case of certain combinations of additives andsintering temperatures, crystalline silicon based oxides or oxynitridessuch as Si₂ N₂ O or Y₂ SiO₇ can also be formed. By changing sinteringatmosphere, temperature and amount and starting composition of theintergranular phase, the type of in situ formed secondary transitionmetal phase can be affected. It is with the purview of the skilledartisan using other raw materials and equipment, taking thermodynamicconsiderations into account, to determine the conditions by experiments.

The sintered body is ground to an insert with the desired shape andsize. The body can be either ground on all surfaces (top, bottom,clearance surface) or on only one, two or three of the surfaces. Theunground surfaces will thus be used as sintered. The completely groundor partly ground or unground insert is either honed or provided with achamfer. Finally, the insert may be provided with a wear resistant layerusing either CVD technique (including MT-CVD) or PVD technique, as knownin the art. The layer thickness should be in the range of 1-20 μm,preferably 1-10 μm, and most preferably 2-7 μm. Preferably, the layershould consist of a 1-7 μm, preferably 1-5 μm, Al₂ O₃ layer and a lessthan 4 μm, preferably 2 μm, thick TiN layer. When the refractory layeris applied, the insert is edge treated to reduce the coating thicknessat the edge and to get a smoother coating surface.

The invention is additionally illustrated in connection with thefollowing Examples which are to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the Examples.

EXAMPLE 1

The silicon nitride (98 weight %, UBE SN-E10) and 1 weight % yttria(H.C. Starck, min. 99.8% purity by weight) and 0.5 weight % alumina(Sumitomo, AKP30) were added together with 0.5 weight % niobium oxide(H.C. Starck chemically pure grade, min. 99.8% by weight), and milledfor 36 hours in water with silicon nitride cylindrical pebbles as amilling media and together with suitable dispersing agents and pressingaids. After milling, the dispersion was sieved and granulated. Thegranulated powder was then pressed uniaxially to green bodies of thedesired geometry.

In order to remove the organic additives, the green bodies were heatedin hydrogen at 600° C. for 4 hours. The presintered bodies were thensintered in a GPS furnace with a sintering temperature of 1850° C. undera nitrogen pressure of 20 bar for one hour under which the materialreached closed porosity. The pressure was then raised to 100 bar andmaintained for one hour. The microstructure of the sintered material canbe seen in the FIGURE. The sintered material consisted of beta-Si₃ N₄grains, about 25% of which were eongated, with an aspect ratio of >3with less than 3 weight % intergranular phase. The porosity was 0.1 %.About 1 weight % of NbSi₂ with an average size of about 0.5 μm wasevenly distributed in the microstructure. The inserts were then groundto SNGN 120412 T02520 geometries and tested in an intermittent cuttingoperation in grey cast iron with cast skin (600 m/min, 0.25 mm/rev,cutting depth 2 mm). The flank wear after 24 minutes is listed in Table1, together with the hardness of the material.

EXAMPLE 2

According to the above-described procedure, Si₃ N₄ (97.65 weight %, UBESN-E10), Y₂ O₃ (1 weight %), Al₂ O₃ (0.5 weight %) and Ta₂ O₅ (0.85weight %) were processed and sintered at 1850° C. under a nitrogenpressure of 20 bar for one hour under which the material reached closedporosity. Then, the nitrogen pressure was raised to 100 bar for anotherhour for the final densification. The sintered material consisted ofbeta-Si₃ N₄ grains, about 20% of which were elongated, with an aspectratio of >5 with <4 weight % intergranular phase. The porosity was 0.1%. Less than 1 weight % of TaSi₂ with an average size of about 1 μm wasevenly distributed in the microstructure. The inserts were then groundto SNGN 120412 T02520 geometries and tested in an intermittent cuttingoperation in grey case iron with cast skin (600 m/min, 0.25 mm/rev,cutting depth 2 mm). The flank wear after 24 minutes is listed in Table1, together with the hardness of the material.

EXAMPLE 3

According to the above-described procedure, Si₃ N₄ (97 weight %, UBESN-E10), Y₂ O₃ (1 weight %), Al₂ O₃ (0.5 weight %) and Nb₂ O₅ (1.5weight %) were processed and sintered at 1850° C. under a nitrogenpressure of 20 bar for one hour under which the material reached closedporosity. Then, the nitrogen pressure was raised to 100 bar for anotherhour for the final densification. The sintered material consisted ofbeta-Si₃ N₄ grains, about 15 % of which were elongated, with an aspectratio of >5 with about 2 weight % intergranular phase. The porosity was0.3 %. About 2 weight % of NbSi₂ with an average size of <1 μm wasevenly distributed in the microstructure. The inserts were then groundto SNGN 120412 T02520 geometries and tested in an intermittent cuttingoperation in grey cast iron with cast skin (600 m/min, 0.25 mm/rev,cutting depth 2 mm). The flank wear after 24 minutes is listed in Table1, together with the hardness of the material.

EXAMPLE 4

According to the above-described procedure, Si₃ N₄ (97 weight %, UBEESP), 1.5 weight % Y₂ O₃, 0.75 weight % Al₂ O₃, 1.0 weight % Nb₂ O₅,0.25 SiO₂ were processed and sintered at 1850° C. under a nitrogenpressure of 20 bar for one hour under which the material reached closedporosity. Then, the nitrogen pressure was raised to 100 bar for anotherhour for the final densification. The sintered material consisted ofbeta-Si₃ N₄ grains, about 20% of which were elongated, with an aspectratio of >5 with less than 5 weight % intergranular phase. The porositywas <0.2%. Less than 1 weight % of NbSi₂ with an average size of lessthan 1 μm was evenly distributed in the microstructure. The inserts werethen ground to SNGN 120412 T02520 geometries and tested in anintermittent cutting operation in grey case iron with cast skin (600m/min, 0.25 mm/rev, cutting depth 2 mm). The flank wear after 24 minutesis listed in Table 1, together with the hardness of the material.

                  TABLE 1                                                         ______________________________________                                        Material      Flank Wear                                                                              Hardness HV10                                         ______________________________________                                        Example 1     0.22 mm   1582                                                  Example 2     0.20 mm   1584                                                  Example 3     0.19 mm   1581                                                  Example 4     0.23 mm   1528                                                  Reference #1  0.35 mm   1443                                                  (Prior Art)                                                                   Reference #2  0.35 mm   1597                                                  ______________________________________                                    

The reference material #1 is a commercial grade of beta silicon nitride(Coromant Grade CC690). The reference material #2 has the same startingcomposition as in Example 1, but without transition metal additives. Ithas also shown a somewhat more brittle behavior.

The results show that a combination of good fracture toughness,thermoshock resistance and wear resistance can be achieved with the saidtype of silicon nitride material. The material thus shows good wearresistance and the capacity to stand mechanical and thermal stresswithout leading to catastrophic failures.

EXAMPLE 5

Inserts with the style SNGN 120412 T02520 were manufactured according toExample 3 above. The inserts were divided into three groups, A, B and C,and a refractory coating consisting of layers of Al₂ O₃ and TiN wasapplied using CVD technique. The layer thicknesses were according to thefollowing in μm:

                  TABLE 2                                                         ______________________________________                                                                 TiN                                                             Al.sub.2 O.sub.3                                                                            (on top of the                                       Insert     (closest to the insert)                                                                     Al.sub.2 O.sub.3 layer)                              ______________________________________                                        5A         4             1                                                    5B         1.5           0.5                                                  5C         reference without refractory coating                               ______________________________________                                    

Finally, all edges of the inserts were treated so as to reduce thecoating thickness and to get a smoother coating surface.

The inserts were tested in a turning operation in nodular cast iron(SS0727) with remaining cast skin (600 m/min, 0.25 mm/rev, 2 mm cuttingdepth). After 3 minutes, the following wear was measured.

                  TABLE 3                                                         ______________________________________                                                     Flank Wear                                                                              Crater Wear Area                                       Inserts      (mm)      (mm.sup.2)                                             ______________________________________                                        5A           0.12      0.09                                                   5B           0.21      0.30                                                   5C           0.25      0.52                                                   ______________________________________                                    

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A method of making a silicon nitride cutting toolinsert by powder metallurgical methods comprising the steps of:a)preparing a silicon nitride slurry by wet dispersion in water or anorganic solvent of silicon nitride powder with powders of yttrium oxide0.1-5 weight %, aluminum oxide 0.1-5 weight % and one or more transitionmetal chosen from the group consisting of: niobium oxide, tantalum oxideand mixtures thereof in an amount of
 0. 1-5 weight %, whereby the totalsum of added oxides is less than 6 weight %, and dispersing agents; b)drying the slurry; c) granulating the dried slurry to form a powder; d)shaping the powder to form a cutting tool insert shape; and e) sinteringthe cutting tool insert shape using a pressure assisted sinteringtechnique to form a sintered cutting tool insert, and in situ forming atleast one secondary crystalline phase comprising at least one carbide,nitride, carbonitride or silicide of niobium, tantalum or mixturesthereof which is nucleated from an intergrannular phase duringsintering.
 2. The method of claim 1 wherein the yttrium oxide is 0.2-3weight %.
 3. The method of claim 1 wherein the aluminum oxide is 0.1-3weight %.
 4. The method of claim 1 wherein the niobium oxide or tantalumoxide or mixtures thereof is 0.2-3 weight %.
 5. The method of claim 1wherein SiO₂ is added in an amount less than 1 weight %.
 6. The methodof claim 1 wherein a wear resistant coating is deposited on the insert.7. The method of claim 6 wherein said coating consists of a 1-7 μm Al₂O₃ layer and a <4 μm thick TiN layer.
 8. The method of claim 1 furthercomprising adding one or more pressing aid, to the slurry.
 9. The methodof claim 1, wherein 0.05 to 3.0 weight % of the secondary crystallinephase is formed.
 10. The method of claim 1, wherein 0.3 to 2.0 weight %of the secondary crystalline phase is formed.
 11. The method of claim 1,wherein the sintered cutting tool insert comprises beta silicon nitride.12. The method of claim 1, wherein the sintered cutting tool insertcomprises elongated beta silicon nitride grains having an aspect ratiogreater than
 3. 13. The method of claim 1, wherein the sintered cuttingtool insert comprises elongated beta silicon nitride grains having anaspect ratio greater than 5.